Legionella pneumophila is the causative agent of a severe pneumonia called Legionnaires' disease. The ability of Legionella to replicate inside of phagocytic cells is central to host pathogenesis and requires a specialized secretion system called Dot/Icm. The focus of this project has been to determine how the Dot/Icm system enables Legionella to create an intracellular vacuole that supports replication. Towards this end, we have demonstrated that this process involves the subversion of host vesicles derived from the endoplasmic reticulum, which are used by Legionella to remodel the plasma membrane-derived organelle it occupies initially into a vacuole that resembles the endoplasmic reticulum. Proteins delivered into host cells by the Dot/Icm system promote this membrane transport pathway. Over the past funding period we have identified multiple bacterial proteins that target the host membrane transport regulator Rab1. To understand how these proteins control membrane transport and promote biogenesis of a vacuole that permits Legionella intracellular growth, we will investigate the function of these proteins during infection of host cells by Legionella. Specifically, we will determine how the biochemical activities of these proteins are regulated spatially and temporally during infection to coordinate the cycling of Rab1 on vacuoles containing Legionella, determine how Rab1 activation on the vacuole containing Legionella can promote the recruitment and fusion of endoplasmic reticulum-derived vesicles, and determine the role host proteins that regulate phosphatidylinositol 4-phosphate play in vacuole maturation.